1. Field of the Invention
This invention relates generally to lids for containers, and more particularly to a pull-open type lid device for wide-mouthed containers such as cans wherein a pull tab of the lid device is manually pulled to open the lid for access the content of the container. The invention also relates to a method of producing such a lid device.
2. Description of the Prior Art
As is well known, a typical lid for wide-mouthed containers such as cans is made of a single metallic plate and formed with a looped weakening groove (a looped line of thin-walled portion) to define an easily openable portion. This openable portion is provided at a position adjacent to the weakening groove, with a metallic pull tab or ring which, when manually pulled up, causes the openable portion to be torn off from the rest of the lid along the weakening line.
Such a metallic lid is disadvantageous in that the respective torn edges of the openable portion and the resulting opening become sharpened, which may cause injury of the lips when drinking the content of the container or injury of the hand when making access into the container. Further, the sharp edge of the openable portion may accidentally come into injuring contact with various portions of the human body.
U.S. Pat. No. 4,674,649 to Pavely discloses a lid device which eliminates the safety problems after opening. Specifically, the lid device of this patent comprises a metallic can end or lid body having an aperture which is closed by a plastic closure member integrally molded in situ. The closure member is integrally formed with a pull tab which is used to tear-open the closure member along a weakening line. The aperture of the can end is defined by a downturned flange, so that no sharp edge is exposed after opening of the closure member which itself is safe because of its material.
However, the lid device according to the above U.S. patent is disadvantageous in the following respects.
First, cheap plastic materials such as polyethylene, which are now widely used and available in the market, have been found to be slightly pervious to vapor. Therefore, if the closure member is made of such a cheap material, the container cannot or should not be used for long storage of a moisture-rejective content such as milled coffee, powdered milk, salt or sugar because the content will be degraded by the environmental moisture getting into the container through the plastic closure member, or it will be excessively dried or lose fragrance by the contained moisture getting out through the closure member. Such a problem becomes more pronounced when the aperture to be closed by the closure member is increased in area.
It is of course possible to eliminate the above problem by increasing the thickness of the closure member to the degree enough to prevent vapor penetration or by using a vapor impervious resin such as polypropylene for molding the closure member. However, the former measure requires an increased amount of the plastic material to result in cost increase. The latter measure is similarly defective in that the vapor impervious resin is more costly than the vapor pervious resin. Further, the vapor impervious resin is relatively hard, resulting in difficulty in opening the closure member.
Secondly, it is very difficult to print various indications or patterns on the outer surface of the closure member which has been once molded in place relative to the lid body can end). In fact, such printing is often necessary to indicate various characteristics of the content or to improve visual attractiveness of the lid device as a whole. However, the only possibility found for the prior art closure member seems to be simultaneous formation of impressed patterns at the time of molding the closure member.